1. Field of the Invention
The present invention relates to an image communication apparatus and an image communication method in both of which communication is allowed with image data and sound data, and a recording medium which stores this method.
2. Description of the Related Art
In an application in which an image and sound are transmitted, such as video telephony, the image and sound are transmitted without coordinating packet sizes therefor.
In packet transmission of sound, a packet as small as possible is employed in order to reduce the feeling of delay at the receiving side by the time required for capturing and packeting sound and, if the sound data is compressed and transmitted, the time required for compression.
Since the start time and the end time of packet transmission shifts between the transmission side and the receiving side according to the time required for packet transmission, the user at the receiving side usually feels that the receiving timing of sound data is natural. The time periods described above such as that required for capturing sound need to be reduced as much as possible.
On the other hand, in packet transmission of an image, high-speed image transmission is enabled by dividing a captured image of one screen into packets as large as possible and transmitting them. Therefore, it is preferred that the packet size for an image be as large as possible.
As described above, image data and sound data have different natures in terms of packet size.
In applications such as video telephony, since the delay of sound is more conspicuous than that of an image at the receiving side, it is usual to give priority to sound data over image data in transmission. A function for not transmitting sound if its level is equal to or less than a threshold is usually provided because it is of no use to packet-transmit sound data without sound.
The upper limit of the size of a packet which can be transmitted is usually specified in packet transmission. This limit is called the maximum transfer unit (MTU) of a network. In the Ethernet, for example, MTU is set to 1500 bytes. Although the maximum packet size is not specified in the Internet, it is recommended that a packet data capable of being handled be 576 bytes in a network and gateway without fragmentation. Since a protocol header is added to data, the actual data size is less than 576 bytes. Such an actual upper limit of data size is assumed to be the maximum packet size.
In the conventional packet transmission in which an image and sound are transmitted with packet sizes suited thereto, if an image to be transmitted with a large packet size and sound to be transmitted with a small packet size are sequentially output to the same transfer line, the sound corresponding to a received image reaches the receiving side with a delay from the time the image was received. In serial transmission of image packets and sound packets, for example, since a sound packet cannot be transmitted until one image packet has been sent, even if a large amount of sound data is to be transmitted (namely, sound is continuously generated), the intervals between a plurality of sound packets, which express continuous data of sound, become large. As a result, a period in which sound is interrupted becomes very large.